1. Field of the Invention
The present invention relates to improving transmitted signal quality in an active phased array antenna utilizing solid state power amplifiers transmitting two or more fundamental communications beams. In particular, selected intermodulation beams arising from nonlinear amplifier operation are nulled to improve signal quality.
2. Discussion of the Related Art
Active phased array antennas include a plurality of radiators driven by respective amplifiers. FIG. 1 shows a prior art active phased antenna 100. The antenna has radiators 120 located at the intersections of lines of a corresponding x-y rectangular grid. Radiators may be located in the grid by reference to an (x,y) coordinate such as (1,1) or (3,3). This two coordinate referencing system is used in some antenna equations. Another coordinate referencing system uses one coordinate, each element being sequentially numbered. For example, in a 3×3 array, element (1,1) becomes element 1 and element 3,3 becomes element 9. This referencing system is used in some antenna equations.
FIG. 2A shows a prior art active phased array antenna 200A. A beam forming section incorporating “i” beam forming elements 250 is coupled with signal(s) 224 and commanded angle inputs 222. Signal(s) with an applied phase shift for beam steering 255 are outputs of the beam forming section and are coupled to the feed chain section incorporating “i” feed chain elements 254. Feed chain section outputs 257 are coupled to “i” radiators 220 of an antenna array 260.
FIG. 2B shows a more detailed version 200B of the prior art active phased array of FIG. 2A. Here, an ith radiator 220 is coupled with incoming signals S1, S2 via an ith antenna beam forming element 204 of beam forming section 250 and an ith feed chain element 205 of feed chain section 254. In this embodiment, a fundamental beam steering processor 202 is common to a plurality of antenna beam forming sections.
As used herein, the term processor refers to a device for processing information. In particular, digital processors such as microprocessors and other digital processing devices are included. Various processor embodiments include one or more processors. And, some processor embodiments include one or more memory device(s) such as semiconductor and/or hard disc drive memory devices and input/output device(s) such as bus communications, parallel communications, and serial communications devices.
Beam forming section inputs include a plurality of signals 224 and their related angles 222. For each signal S1, S2, two angles, commanded elevation θ0 and azimuth φ0 determine the direction of the beam carrying the signal and therefore the intended receiver of the signal. For example, a first fundamental beam might be directed to a receiver in a first city at the angle pair (θ0, φ0) and a second fundamental beam might be directed to another receiver in another city at the angle pair (θ′0, φ′0). Manipulating the direction of a communication beam is sometimes referred to as steering the beam.
Beam forming entails creation of a phase front for each beam that is normal to the desired direction of the beam. These phase fronts are created by appropriately shifting the phases of the incoming signals S1, S2 in beam forming elements 204. Each one of “i” antenna beam forming elements includes steering phase shifters PSi1, PSi2 that create corresponding shifted signals Si1a, Si2a. In various embodiments, the phase shifters include one or both of digital and analog phase shifters.
Phase shifts Zi1, Zi2 are applied to the signals S1, S2 to create shifted signals Si1a, Si2a. In an embodiment, the phase shifts are calculated within the fundamental beam steering processor 202. And, in an embodiment, these applied phase shifts are functions of uniform progressive phases αx, αy as shown in equations 1a,b below.Zi1=q1(α1,x, α1,y)   Equation 1aZi2=q2(α′1,x,α′1,y)   Equation 1b
As shown in equations 2a-d below, the uniform progressive phases αxx, αy are determined by the commanded beam angle pairs θ0, φ0 and θ′0, φ′0.
                              tan          ⁢                                          ⁢                      ϕ            0                          =                              α                          1              ,              y                                            α                          1              ,              x                                                          Equation        ⁢                                  ⁢        2        ⁢                                  ⁢        a                                                      sin            2                    ⁢                      θ            0                          =                                            α                              1                ,                x                            2                        +                          α                              1                ,                y                            2                                                          (                              2                ⁢                                                                  ⁢                π                ⁢                                                                  ⁢                                  d                  /                  λ                                            )                        2                                              Equation        ⁢                                  ⁢        2        ⁢                                  ⁢        b                                          tan          ⁢                                          ⁢                      ϕ            0            ′                          =                              α                          1              ,              y                        ′                                α                          1              ,              x                        ′                                              Equation        ⁢                                  ⁢        2        ⁢                                  ⁢        c                                                      sin            2                    ⁢                      θ            0            ′                          =                                            α                              1                ,                x                            ′2                        +                          α                              1                ,                y                            ′2                                                          (                              2                ⁢                                                                  ⁢                π                ⁢                                                                  ⁢                                  d                  /                  λ                                            )                        2                                              Equation        ⁢                                  ⁢        2        ⁢                                  ⁢        d            
Note, equations 2a-d assume dx=dy=d. This assumption simplifies the analysis and the equations.
Phase shifter outputs Si1a and Si2a are combined and amplified in the ith feed chain element 205 that includes a signal combiner 210 and a solid state amplifier 212. The signal combiner 210 is coupled to the input signals Si1a, Si2a and its output 211 is amplified in the amplifier. The ith radiator element 220 is coupled to the amplifier 212 via an amplifier output 213.